Nowadays, there is an increasing interest to replace conventional incandescent bulbs with LEDs (light emitting diodes). Nevertheless, how to make LED driving apparatus be compatible with dimmers in existing lighting apparatuses becomes a challenge.
The most common dimmers are phase cut dimmers including leading edge dimmers and trailing edge dimmers, wherein the main supply is cut off for part of the main cycle. In some situations, the dimmer has been removed so there is no dimmer. To sum up, there are three different dimming conditions: leading edge dimming, trailing edge dimming and no dimming.
A bleeding circuit is often employed to improve dimming performance of the LED driving apparatus. A prior art bleeding circuit including a bleeding resistor Rp and a bleeding transistor Sp is shown in FIG. 1. The bleeding transistor Sp turns on to provide a bleeding current when the bus voltage Vbus is smaller than a predetermined value.
Since the bleeding circuit shown in FIG. 1 is coupled between the output terminal of the rectifier bridge and the reference ground, the bleeding transistor Sp therein need to sustain a high voltage, which makes it expensive and hard to be integrated.
Moreover, due to the significantly different working principles, the bleeding current required in trailing edge dimming is much larger than that needed in leading edge dimming. As a result, the bleeding resistor Rp is generally designed to be relatively small to fit both leading edge dimming and trailing edge dimming, which absolutely limits the efficiency of the LED driving apparatus in leading edge dimming. In no dimming situations, the efficiency suffers even more.